Q.Draw qualitatively energy-level diagram showing d-orbital splitting in the octahedral environment. Predict the number of unpaired electrons in the complex [Fe(CN)6]4- . Is the complex diamagnetic or paramagnetic? Is it coloured? Explain.

(a) $Ti^{3+}$ ($3d^1$) shows a d-d transition and is coloured, while $Sc^{3+}$ ($3d^0$) has no d electrons and is colourless; chrome plating electrodeposits a protective/decorative chromium layer. (b) Werner's theory rests on primary and secondary valencies, the latter being fixed in number and direction, giving the complex its geometry.

(a)(i) Colour of $[Ti(H_2O)_6]^{3+}$ vs $[Sc(H_2O)_6]^{3+}$:

In $[Ti(H_2O)_6]^{3+}$, titanium is present as $Ti^{3+}$, with configuration $[Ar]3d^1$ - it has one unpaired electron in the d subshell. The six surrounding water ligands split the five degenerate d-orbitals into two sets of different energy (crystal field splitting, $t_{2g}$ lower, $e_g$ higher, in an octahedral field). The single d-electron can absorb a photon of visible light of just the right energy to jump from the lower ($t_{2g}$) to the higher ($e_g$) set - this is a d-d transition. Since some wavelengths of white light are absorbed for this transition, the complementary wavelengths are transmitted/reflected, and the complex appears coloured (violet).

In $[Sc(H_2O)_6]^{3+}$, scandium is present as $Sc^{3+}$, with configuration $[Ar]3d^0$ - there are no d-electrons at all. With an empty d subshell, no d-d electronic transition is possible on absorbing visible light, so the complex is colourless.

(a)(ii) Chrome plating: see the note below - chrome plating is the electrolytic deposition of a thin, hard, corrosion-resistant, decorative layer of metallic chromium onto a base-metal object (usually already given a copper and nickel undercoat for smoothness and adhesion). The object is made the cathode in a bath of chromic acid ($CrO_3$) in dilute sulphuric acid, with an insoluble lead-alloy anode; passing current deposits chromium onto the article, giving it wear resistance, corrosion resistance and a bright finish (used on automobile parts, taps, bathroom fittings, etc.).

(b) Postulates of Werner's theory of coordination compounds:

1. Metals in coordination compounds exhibit two types of valency: a primary (ionisable) valency, satisfied by negative ions and equal to the oxidation state of the metal, and a secondary (non-ionisable) valency, satisfied by negative ions or neutral molecules (ligands) and equal to the coordination number of the metal.
2. Every metal ion has a fixed number of secondary valencies (a fixed coordination number, e.g. 6 or 4), which it always tries to satisfy.
3. The secondary valencies are directed towards fixed positions in space, and it is this spatial arrangement that gives the complex its characteristic, definite geometry (e.g. octahedral for coordination number 6, tetrahedral or square planar for coordination number 4) - this directional character of the secondary valency is what explains the stereochemistry (isomerism) of coordination compounds.